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One special thing about ReactDOM.render is that it only updates DOM elements that have changed.

That means that if you render the exact same thing twice in a row, the second render will do nothing:

varhello=<h1>Hello world</h1>;
// This will add "Hello world" to the screen:ReactDOM.render(hello, document.getElementById('app'));
// This won't do anything at all:ReactDOM.render(hello, document.getElementById('app'));

This is significant! Only updating the necessary DOM elements is a large part of what makes React so successful.